1. Field of the Invention
The present invention is related to a method for forming through-holes in a printed wiring board.
2. Prior Art
There are the following conventional methods for forming through-holes by filling a conductive material in the through-holes passing through the printed circuits in a printed wiring board. That is, a formation method in which a conductive material is deposited on filling pins and thereafter the pins are inserted into the through-holes made in the printed wiring board, thereby to deposit the conductive material deposited on the circumference of the filling pins inside the through-holes, and a method in which a silk plate for print corresponding to the circuit design of a printed wiring board is made and a conductive material is filled in the individual through-holes of the printed wiring board through the silk plate.
However, in the former method using pins of the conventional methods for forming through-holes, the amount of the conductive material deposited on the filling pins is absolutely small, and to fill the inside of the through-holes with the conductive material, a work of depositing the conductive material on the filling pins and a work of inserting the pins after deposition into the individual through-holes of the printed wiring board must be repeated a plurality number of times, so that this method has a disadvantage of low working efficiency in addition to the cumbersome work.
In addition, in the above work, the conductive material deposited inside the through-holes by the filling pins must be hardened each time, and thus the thermal damage to the material of the substrate of the printed wiring board is significant, so that there is a defect that the size stability of the printed wiring board is remarkably degraded.
Moreover, many pins are necessary at a time if the number of the through-holes of the printed wiring board is great, and usually it is impossible to use more than 1000 pins and the number of pins is restricted in terms of design, which results in a restriction on the size of the printed wiring board (usually the size larger than 300 m/m.sup.2 is impossible).
Accordingly, the facilities and work processes such as the manufacture of a pin jig having pins arranged in correspondence to the individual through-holes of a printed wiring board are difficult both industrially and in the point of work, and there is a drawback of poor mass production capability.
Also, with the silk screen method which is the latter conventional formation method, there is unevenness in the squeegee pressure against the screen and it is difficult to completely fill the conductive material in the through-holes of the printed wiring board with the ordinary squeegee pressure, and for instance, the filling is impossible if the board thickness of the printed wiring board is thick or the diameter of the through-holes is small. (For instance, in the state of the art, the limit of the diameter of the through-holes is 0.7 mm if the board thickness is 1.6 mm.)
And, with such method, the fluidity of the conductive material has a large effect on the evenness of the filling, and the complexity of the work further increases. Accordingly, usually a solvent is added into the conductive material to uniformize the fluidity of the conductive material, but on the other hand, the solvent is vaporized in the through-holes by heating when the conductive material is hardened after the filling, thereby to cause a collection of air bubbles and cracks, which result in conduction failure or the like, and thus there is a large problem in removal of the solvent.
That is, a solvent of the higher alcohol system which has a high boiling point and is difficult to vaporize is used as the solvent to maintain the fluidity of the conductive material, and a temperature at which no air bubble occurs and time are required to completely vaporize the solvent for complete removable thereof, which affects the reliability of the printed wiring board.